In the semiconductor industry, dispensing systems are commonly used for dispensing finite volumes of fluid material. Dispensing systems are deployed in applications such as underfill, encapsulation, supplying adhesives and die attachment.
One conventional method of dispensing uses syringe dispensers which apply time pressure dispensing or dispensing using volumetric valves. A disadvantage of using a syringe dispenser is the problem of stringing or sticking of a bead of the fluid material to a nozzle, which adversely affects the ability of the delivery system to dispense precise, quantitative amounts of fluid material. Stringing is likely to occur at lower pressures, such as when the pressure in the syringe is ramping up or ramping down. For the same reason, stringing also occurs frequently as dispensing time gets shorter.
Jet dispensers may overcome the problem of stringing. In jet dispensing, pressure is generated to act on finite volumes of fluids so that the fluids can be ejected at a high outflow velocity before being stopped with high deceleration. A droplet is then formed and dropped onto a substrate. A single or a continuous stream of micro-droplets from the nozzles can be dispensed in this manner. As nozzles of the jet dispensers do not contact the substrates during jet dispensing, stringing does not occur. This improves reliability and repeatability of dispensing. Jet dispensers may also allow for dispensing finite volumes of fluids with high accuracy. Further, dispensing with jet dispensers is faster since the dispensers do not have to raise their nozzles to break up fluid filaments. Therefore, jet dispensing provides greater flexibility for varied applications.
Jet dispensing can be conducted using actuators which produce thermal pulse, impact force or pressure waves. Pressure waves may be generated by thermal ink jets, piezoelectric vibration actuated jets or mechanical jets. Thermal ink jets create nucleate boiling by heating a resistor to form a bubble on the resistor. When power is turned off, the surrounding ink cools the bubble till it collapses. The actions of forming a bubble and subsequent collapsing of the bubble have an impact on the momentum of the fluid directly above the bubble. Thermal ink jets work well with fluids of low viscosity of, for example below 30 cps.
Piezoelectric vibration actuated jets work by rapidly changing the volume of a fluid chamber to generate acoustic pressure waves. As the pressure waves propagate, the nozzle dispensing pressure will be changed between positive and negative quickly. Thus, the droplet can be formed and ejected out. This technology is limited primarily to fluids with low viscosity.
Mechanical jets use a needle piston held against a nozzle seat. The fluid in the chamber is under low pressure. Pressure is applied to lift the needle piston off the nozzle seat. After a prescribed time, the pressure is released and the needle piston plunges at a controlled rate. When the piston hits the nozzle seat, pressure of the fluid directly between the seat and the piston increases so rapidly that a jet of fluid can be extruded from the nozzle. The impact also generates a shock wave which snaps the fluid from the nozzle.
There are various ways to drive the needle piston. Spring, piezoelectric and magnetostrictive actuators are all suitable actuators to generate a quick and strong impact force. Mechanical jets create very high local pressure at the nozzle and may therefore dispense fluids of high viscosity.
One prior art which uses magnetostrictive material as an actuator is U.S. Pat. No. 5,558,504 entitled “Magnetostrictive Pump for Applying Pastes and Adhesives”. In this prior art, a piston is connected to a magnetostrictive rod. The piston is displaced when the magnetostrictive rod changes in length in response to a changing magnetic field generated by an electric current flowing through a coil around the rod. Rapid dispensing can be achieved using such magnetostrictive actuators, but as the piston stroke is limited by the low strain of the giant magnetostrictive material, the outlet velocity of fluid is not high enough to dispense highly viscous fluid. Additionally, a two valves volumetric system is used in this prior art whereby an upper valve controls the supply of fluid and a lower valve maintains the volume of fluid in the fluid chamber. The action of such a two valves system affects the accuracy of the dispensing volume and the dispensing speed.
Another prior art is U.S. Pat. No. 6,508,196 entitled “Device for Applying Drops of a Fluid on a Surface” which discloses the use of a high speed piston to generate pressure waves for ejecting droplets of fluids. Unlike U.S. Pat. No. 5,558,504, this prior art does not use a two valves volumetric system. Nevertheless, this dispenser cannot dispense highly viscous fluid. Furthermore, the working distance between a nozzle and a substrate has to be less than 0.5 mm, which limits flexibility.
Yet another prior art U.S. Pat. No. 5,747,102 entitled “Method and Apparatus for Dispensing Small Amounts of Liquid Material” discloses a mechanical jet dispenser using a piston actuated by an air solenoid. This dispenser can dispense adhesives with high viscosity but the stroke motion of the piston requires manual adjustment. Furthermore, when the piston hits the nozzle seat, the impact may damage the structure and a loud noise is also produced. The system is inflexible and the parameter window is narrow. Likewise, jet dispensers actuated by electric solenoids have the same problems as dispensers actuated by air solenoids.
Therefore, it would be desirable to devise a dispensing system which is capable of dispensing fluids of high viscosity, at a high frequency and in large quantities when required.